This invention relates to a DC/AC converter for igniting and feeding a gas and/or vapor discharge tube, the converter having two input terminals which are interconnected through a first series arrangement at least comprising a first controlled semiconductor switch element, a load circuit which--in the operating condition--is to include the discharge tube, and a first capacitor, the load circuit together with at least the first capacitor being shunted by a second controlled semiconductor switch element and provided with a circuit element for shunting the discharge tube, a second capacitor is connected parallel to at least the first controlled semiconductor switch element and in series with the first capacitor. The converter is provided with a starting circuit which is connected to a control electrode of one of the controlled semiconductor switch elements and which comprises a second series arrangement of a starting capacitor and a threshold element, and provision is further made of a charge circuit--comprising a resistor--of the starting circuit.
A known DC/AC converter of the kind mentioned is described, for example, in U.S. Pat. No. 4,525,648 (6/25/85). The starting of this converter takes place in that--after the two input terminals of the converter are connected to a direct voltage source--the starting capacitor will be charged to a voltage which is substantially equal to the threshold voltage of the threshold element. This threshold element then in fact breaks down and supplies a pulse to the control electrode of one of the controlled semiconductor switch elements. As a result, this switch element begins to conduct. A current will then flow through this switch element and the load circuit. Subsequently, this switch element becomes non-conducting and the other switch element becomes conducting. This process is then continuously repeated. This leads to an oscillation, i.e. an alternating current through the load circuit including the discharge tube.
A disadvantage of the known DC/AC converter is that, when repeatedly switching on at an output voltage of substantially constant effective value, the starting pulse at the control electrode of the relevant semiconductor circuit element invariably appears a fixed time interval after switching on of the voltage source. This means that this starting pulse may also appear at a premature instant, i.e. if the process of starting the converter cannot be followed by a further oscillation. This disadvantageous situation--which may be accompanied by flickering of the discharge tube--may arise, for example, if during the occurrence of the starting pulse the first capacitor--and/or the second capacitor--has received only a small electrical charge.